The present invention relates to an apparatus and method for measuring the frequency of a pulse signal.
A Japanese publication "Patent Pulse Circuitry Technological Dictionary" (Tokkyo Pulse Kairo Gijyutsu Jiten) published on May 20, 1980 by Kabushiki Kaisha Ohm sha and edited by Yasuo Suzuki and Takehisa Higuchi (Page 520) exemplifies one of such pulse signal frequency measuring apparatus.
The pulse signal frequency measuring apparatus disclosed in the above-identified Japanese publication document includes a counter, an output register, and a timing controller.
The counter counts the input number of pulses within a predetermined counting interval of time. The counting interval of time is prescribed by means of the timing controller. In addition, the counting interval of time is set to a time interval required for a determined frequency measurement accuracy. The result of counting by the counter is latched and outputted in the output register at a time when the timing controller outputs a latch signal to the output register. At the same time, the timing controller outputs a reset signal to the counter to reset the counter. That is to say, the counting result Cn, i.e., the number of pulses counted during the predetermined counting time interval T.sub.n, is outputted from the output register while the counter counts the number of pulses during the next counting time interval, i.e., the time T.sub.n+1. In this way, the output contents of the output register is updated for each counting interval of time T (=T.sub.n =T.sub.n+1 =T.sub.n+2 . . . ). An interval for which the contents of the output register is updated, i.e., an output refreshing interval, is the counting interval of time T required to determine the frequency measurement accuracy.
Therefore, in a case where the above-described pulse signal frequency measuring apparatus is applied to an apparatus for indicating a vehicle speed in an analog form (speedometer), the counter counts the number of pulses derived from a vehicle speed sensor and a pointer of the speedometer is swung through an angle according to the output contents of the output register.
However, since, in this case, a period of time for moving an indicated position of the pointer becomes long as the above-described output refreshing interval becomes long, the output contents of the output register cannot follow up a change in the vehicle speed when the vehicle speed abruptly changes. Consequently, an indicated value of the pointer of the speedometer changes stepwise and does not change smoothly making it difficult for a vehicle driver to recognize the vehicle speed.
Such a problem as described above will be described in detail below.
Suppose that a maximum speed indicated value of the speedometer is 180 Km/h, the angle through which the pointer swings to indicate 180 Km/h is 270 degrees, and a required speed resolution is 0.3 degrees. In this case, a full scale of the speedometer needs to be divided into 270/0.3 =900 to indicate the vehicle speed. In addition, suppose that a pulse repetition rate of a digital vehicle speed sensor is about 500 Hz at the maximum speed 180 Km/h.
In the above-specified vehicle speed indicating apparatus, the time required to count 900 pulses at the maximum speed of 180 Km/h is 900.times.1/500=1.8 seconds. If the above-described time of 1.8 seconds is set as the counting interval, 1.8 seconds is the output refreshing interval of time for the output register. Hence, since the indicated value of the pointer of the speedometer changes after a period of 1.8 seconds, so that the movement of the pointer becomes extremely jerky or intermittent.
It is noted that since a duty ratio of each pulse derived from the vehicle speed sensor is approximately 50%, the countable frequency from the vehicle speed sensor for the counter can be changed to 1000 Hz if the counter counts the number of pulses on each rising or trailing edge of the pulse signal from the vehicle speed sensor. However, in this case, the counting interval of time T, i.e., refreshing interval becomes 0.9 seconds and it is insufficient to eliminate intermittent movement of the pointer.
A change of the indicated value by the pointer having a period of about 0.1 seconds gives a general feeling that the pointer smoothly moves. However, if the output refreshing interval is merely shortened, the counting interval of time T becomes simultaneously short. Consequently, the number of pulses counted during the counting interval of time becomes less and a resolution of the indicating apparatus becomes reduced. In the above-described example, if the counting interval of time T is set to 0.1 seconds, the number of pulses countable within 0.1 seconds is reduced from 900 to 50 at 180 Km/h and the resolution is extremely reduced to 270/50=5.4 degrees (180/50=3.6 Km/h).
Next, another pulse signal frequency measuring apparatus using a so called staggar ring method will be described below.
In the staggar ring method, n counters need to be connected in parallel to one another if a frequency measuring interval of time required from its resolution is T and the output refreshing interval of time is T/n. In detail, if a count value of each counter counted at the time interval T/n which is the same as the output refreshing interval of time is expressed as D.sub.j (j=1 to n) and an output data from the output register at a certain time is expressed as C.sub.i (i=1 to n), the output count data C.sub.i can be derived as follows: ##EQU1##
Thus, the output data C.sub.i is outputted for each output refreshing interval T/n.
In other words, the old count value before (n+1) time is sequentially deleted and in place of (n+1) time count value the latest count value is added to the count value.
Hence, since the sum of the count values in n number of times is outputted after the lapse of the frequency measuring interval T, the count value C.sub.i for each refreshing interval T/n indicates a value proportional to the frequency of the measured pulse signal.
Although in the staggar ring method the data C.sub.i can be refreshed for each desired output refreshing interval, in general n=9 or nearly 9 and therefore the number of parallel counters is accordingly increased. In addition, a multiplexer to select sequentially the output data of the counters needs to be added. Consequently, the construction of the whole circuitry becomes large-sized and complicated.